Jeq50301 611..616

Managed drainage ditches are common in the midwestern United States. These ditches are designed to remove water from fields as quickly as possible, and sediment buildup necessitates dredging, to ensure adequate water removal. This laboratory study was conducted to determine the impact of ditch dredging on soluble phosphorus (P) transport. Ditch sediments were collected from a drainage ditch in northeastern Indiana immediately before and after dredging. The sediments were placed in a stream simulator, and stream water was loaded with 0.55 mM P for 5 d (adsorption experiment). Water was then removed, and ‘‘clean’’ water (no P added) was used for a desorption experiment, lasting 1 d. During the adsorption experiment, pre-dredged sediments were able to remove P from the water column quicker, and P concentrations 120 h after introduction of high P water were lower for the pre-dredged sediments (0.075 mM P) than the dredged sediments (0.111 mM P). During the desorption experiment, P was released to the water column slower in the pre-dredged treatment than the dredged treatment (instantaneous flux at t5 0 was 0.205mM P h for pre-dredged and 0.488mM P h for dredged). This occurred despite higher Mehlich 3–extractable P in the pre-dredged sediments than the dredged sediments. Equilibrium phosphorus concentrations (EPCo) were lower in the pre-dredged sediments during both adsorption and desorption experiments. Transport of soluble P immediately after dredging will likely increase in drainage ditches; however, dredging is a necessary management tool to ensure adequate discharge of water from surrounding fields. ALGAL PRODUCTION in many U.S. surface waters is limited by phosphorus (P) concentrations, and thus small inputs of bioavailable P from anthropogenic sources can result in accelerated eutrophication (Schindler, 1977). Agriculture has been identified by theUSEPA as one of the primary contributors of nonpoint-source P to surface water (USEPA, 1990). A large amount of research has been conducted on ways to decrease or eliminate P discharges from agricultural lands. Efforts to decrease P losses at the field scale have included treatment of manures with chemicals (Shreve et al., 1995; Smith et al., 2004a), modification of animal diets (Smith et al., 2004a, 2004b; Maguire et al., 2003, 2004), and treatment of soils with wastewater treatment residuals (Rhoton and Bigham, 2005). Recent research has been used to identify sites vulnerable to P losses and has placed restrictions on application of manure or fertilizers to those sites based on risk assessment tools collectively known as the P index (Sharpley et al., 2003; DeLaune et al., 2004). Despite these efforts, sufficient P is still discharged to surface waters to induce anthropogenically accelerated eutrophication (Schindler, 1977; Carpenter et al., 1998). Once P reaches surface water, the P in the water column interacts with sediments, such that the sediments can be a source or sink for water column P. Haggard et al. (2004) found that point sources were still important contributors to P in streams, and that chemical amendments could increase P buffering capacity and decrease readily exchangeable P in the sediments. Chemical treatments also changed the physiochemistry of the sediments such that they were a stronger sink of P in the water column than untreated sediment (Haggard et al., 2004; Smith et al., 2005). Runoff from soils fertilized with poultry or dairy manure can increase stream P concentrations, and increase labile P and equilibrium phosphorus concentrations (EPCo) of sediments (McDowell and Sharpley, 2003). In laboratory experiments using simulated streams, the dissolved P in the water column was related to the EPCo of sediments taken from several sites in a central Pennsylvania stream (McDowell and Sharpley, 2003). When sediment samples were collected from seven sites in three tile-fed drainage ditches in northeastern Indiana, sediments from four sites acted as sinks for P in the water column, and sediments in the remaining three sites acted as sources of P to the water column (Smith et al., 2005). The most significant change in sediment dynamics between sites within a single ditch was observed where a confined animal feeding operation (CAFO) was located between the two sites. Sediment samples collected above the CAFO acted as a P sink, while the sediments below the CAFO were contributing P to the water column. When aluminum sulfate and calcium carbonate chemical treatments were applied to the sediments, all sites shifted to P sinks in the water column (Smith et al., 2005). Chemical treatment has been shown to be effective in lakes and reservoirs (Welch and Schrieve, 1994; Kopacek et al., 2000; Haggard et al., 2005); however, this technology has not been extensively tested in streams or ditches to determine the longevity of in situ efficacy. Dredging is a common practice to remove sediments from ditches, streams, rivers, or estuaries, and is generally performed to optimize the flow of water or to ensure adequate water depth for boats or ships, or is an artifact of shell fish harvests. Field experiments involving dosing of streams with high levels of P indicated that sediments were able to remove P from the water column D.R. Smith, E.A. Warnemuede, and C. Huang, USDA-ARS, National Soil Erosion Research Laboratory, 275 Russell Street, West Lafayette, IN 47907. B.E. Haggard, USDA-ARS, Poultry Production and Product Safety Research Unit, Fayetteville, AR 72701. Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the USDA and does not imply its approval to the exclusion of other products that may be suitable. Received 4 Aug. 2005. *Corresponding author ([email protected]). Published in J. Environ. Qual. 35:611–616 (2006). Technical Reports: Surface Water Quality doi:10.2134/jeq2005.0301 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: EPCo, equilibrium phosphorus concentration; PSR, phosphorus sorption ratio. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 611 Published online March 1, 2006